Means the amount payable (in other words, the price of the order placed by the client). The format of presentation – dot-delimited digits. For example 123.45.

The amount should be denominated in RUB.

However, if the prices are denominated (e.g.) in USD on your website when issuing the invoice you need to specify the amount converted from USD to RUB. 

(see Optional Parameters OutSumCurrency).